During differentiation and oncogenic transformation the structures of complex carbohydrates in the cell change. Many monoclonal antibodies which detect differentiation or cancer- associated antigens are directed against these carbohydrates. Recently, more cancer-associated carbohydrate antigens were characterized. Antibody MOV2 binds the Le(a) oligosachride, whereas antibody ONC-M26 binds the SLe(x) heptasacharide. The latter antibody also strongly binds a novel disialylated Le(x) glycolipid. Another antibody, MOV15, detects difucosylated type 2 chain oligosacharides (Le(y)-active) on mucins elevated in the serum of cancer patients. Other antibodies are also useful for studying the function of specific carbohydrate sequences. For example, antibody LeoMe13 binds strongly to ganglioside GD2 and with lesser affinity to gangliosides GT3, GD3 and GQ1b. This antibody binds melanoma cells and specifically blocks their killing by anomalous killer (AK) cells but not by classical cytotoxic T lymphocytes (CTL) or natural killer (NK) cells. Thus, human anomalous killer cells may recognize and use these carbohydrate tumor markers as targets to kill melanoma cells. Other carbohydrate antigens may be used to study changes in development and function of specific glycoproteins. For example, the neural cell adhesion molecule, N-CAM, is a transmembrane glycoprotein that mediates adhesion among normal and tumor cells of neuroectodermal origin. Monoclonal antibodies produced against the polysialic acid of the capsular polysaccharides of Menningococcus B bacteria can distinguish the embryonic from the adult form of N-CAM. As the embryo develops into an adult, the length of polysialic acids on N-CAM decreases. Removal of sialic acid increases the adhesion among N-CAM molecules, suggesting that the developmental regulation of these oligosaccharides modulates the function of N-CAM.